Fixation of prosthetic flexible tension members, such as tendons or ligaments, to relatively rigid structures remains a difficult problem. A notable example is the use of artificial ligaments, such as the Leeds-Keio anterior cruciate ligament replacement in the knee. In that example, surgeons using the conventional surgical procedure of bone fixation—drilling a hole in the tibia, inserting the ligament, and securing with a suture or pin—have reported instances of fragmentation of the polyester fibers of the prosthesis within a few months to a few years. A compression plate has also been conventionally used, whereby tension members are cut and the ends are secured between two plates that may be textured and held together by compression screws. While this allows greater control of local stress concentration than the simple bone-hole, in theory the compression plate delivers extremely high shear stresses to the tension member locally, which may cause fatigue failure and breakage over a number of stress cycles.
A knob-loop fixation device has been described previously (pending patent application Ser. No. 12/678,008) to address the stress-concentration issue, but requires a substantial thickness that may be disadvantageous. Such thickness may be problematic in some cardiac, plastic, reconstructive, or orthopaedic surgical applications, particularly in regions where the skin is quite close to the bone (e.g., the frontal bone in the case of a cosmetic surgical “brow lift” or the olecrenon in an orthopaedic surgical elbow prosthesis).
Further, complications occur in that the surface to which the fixation device is attached may vary in its contour. Therefore a thinner fixation device having sufficient flexibility to allow a finite number of size/shape models conformable to anatomical variations would be of benefit. Further, a structure having a soft flexible interface to fibers (reducing stress concentration) with a harder external surface (to interface with other tissues) would also be advantageous.
Natural tendon ends, which are living tissue, have been conventionally connected to a “towel bar” fixture on artificial bones, over which the tendons are looped and sewn. Because of the shape of tendons—generally flattened in the plane of attachment, the axis of curvature is generally perpendicular to the surface to which the tendons are to be attached. To avoid intolerable protrusion into surrounding tissue structures, the radius of curvature is generally small. Since the compressive stress on the surface of a tension member about any rod or pulley, is directly proportional to the tension applied and inversely proportional to both the radius of curvature and the projection of contact surface perpendicular to the transmitted tension, compressive forces that are intolerable by the tension member may be generated. However, an artificial force transmitting tension member, such as an artificial tendon, may be formed in any cross-sectional configuration. This allows the looped portion to be relatively thin, flat, and oriented in the plane of the surface to which the tension member is to be attached.
Further, some clinical situations are anticipated where the following improvements may increase the versatility or practicality of such devices: (1) alternative or supplemental methods of adjusting the length of and/or tension in a repair or surgical construct by a fixation device, (2) a reduction in the thickness of the hardware in a medial portion of the fixation device, and (3) an elongated tension element configured to transmit a force from a muscle to a distant location that preserves a continuity of the prosthetic fibers from within the muscle to the distant location and thus avoids mechanical weaknesses of otherwise required connections.